Ultrasonic detection systems are well known for the protection of buildings, warehouses, and other areas. The systems operate by having one or more transmitters which radiate ultrasonic soundwaves into an area to be protected. The ultrasonic waves are reflected by objects in the protected area, and these "echoes" are received by a receiving transducer which produces electrical signals in response thereto. The presence of intruding objects may be detected by monitoring the received echoes for doppler-shifted frequency components which are produced by the reflection of ultrasonic waves from a moving object.
The output signal from the receiving transducer is applied to electronic signal processing circuitry which detects the presence of a doppler-shifted echo indicative of an intruder. Usually the processor includes circuitry to discriminate between echoes caused by an intruder and echoes caused by movement of other objects in the protected area which would otherwise cause false alarms, such as flapping curtains and the like. Additional circuitry may also be included to discriminate against ambient noise in the frequency band being used.
It is frequently desirable to be able to protect large areas or several different areas with a single ultrasonic intrusion alarm system by providing a plurality of ultrasonic transducers and a plurality of ultrasonic receivers located in the areas to be protected. Since previous multiple transducer systems have required relatively complex and expensive circuitry to process the receiver output signal to detect an intruder, multiple transducer systems are usually implemented by applying all the receiver outputs to a single processor. Since only one relatively complex processor is required in such a system and since both the transmitting and receiving transducers are relatively inexpensive, several transmitters and receivers may be used to economically provide protection for a large area or a number of individual areas. However, such systems have the disadvantage that the processor is unable to distinguish between signals from different transducers. The only output that such a system can provide is that an intruder is somewhere within the protected areas. The processor is unable to indicate where the intruder is located within the protected area.
To provide a precise indication of which receiver has detected an intruder, a processor for each area and/or each receiving transducer may be used. However, current methods of processing the transducer output signals require relatively complex electronics, and providing such a processor for each of a plurality of receivers is generally too expensive to be practical, except in more critical installations.